Traditional approaches to multimedia streaming provide content that is encoded at a constant bit rate and transmitted from a server to one or more clients over a constant bit rate channel. For example, standard telephone quality audio content is often encoded at 64K bits per second (b/s). Therefore, a telephone transmission channel must have a throughput of at least 64K b/s to properly stream the audio content.
In contrast, it is sometimes more advantageous to encode video content using a variable bit rate, the reason being that for certain types of video content—such as movies—the information that needs to be encoded varies over time. An idle segment of a movie contains less information than a segment where there is a great deal of action. It may be inefficient to encode such content at a constant bit rate. Depending on the bit rate chosen, idle segments can under-utilize an available bit rate, while action segments may not have sufficient bit rate to be encoded with the same quality as the idle segments.
Encoding a movie at a bit rate that varies as necessary to obtain the desired quality allows the movie to be encoded at a constant quality. However, variable bit rate content can pose problems for streaming media applications. One of the main problems is that the throughput of the transmission channel is often limited. If the instantaneous bit rate of the encoded content is ever higher than the throughput of the channel, the content cannot be streamed, even if the average bit rate of the content is less than the channel's throughput.
Another problem encountered is that transmission channels sometimes require that a constant bit rate be reserved for the streaming content application. In such cases, the channel will not be utilized efficiently. To stream the content, the reserved bit rate must be at least as large as the peak bit rate of the content. That means that during periods when the bit rate of the content does not fully reach the peak, a portion of the reserved bit rate is not being utilized, and that too much bandwidth was reserved in the first place.